1. Field of the Invention
The present invention relates to a method and an apparatus for treating a surface of a rotating thin plate (hereinafter referred to as wafer) by supplying a surface treatment solution (such as an etchant or a developing solution) thereto. The wafer may be a semiconductor wafer, a glass wafer for a photomask or a glass wafer for a liquid crystal display device.
2. Background of the Prior Art
In recent years, photoetching technology has become very important. For instance, in semiconductor industries, a microscopic circuit pattern cannot be formed on a silicon wafer without using photoetching technology.
In general, photoetching is effected as follows: A photoresist layer is formed on a layer to be etched for the purpose of protecting the layer to be etched from an etchant. A pattern (which is also formed on a photomask by photoetching) is optically printed on the photoresist layer. The photoresist layer is developed, whereby the pattern on the photomask is transferred onto the photoresist layer. When the etchant is supplied from above the photoresist layer, portions of the layer to be treated (i.e., portions not protected by the photomask) are etched. After the layer to be treated is sufficiently etched, the remaining photoresist layer is removed. Thus, the same pattern as that on the photomask is formed on the layer to be treated.
As is evident from the above-mentioned brief description of the photoetching process, it is necessary to treat the surface of the wafer on various occasions. For these purposes, it is necessary to supply an etchant, a developing solution or the like to the surface of the wafer. The present invention is directed particularly to an improved technique for treating a surface of a wafer while rotating the wafer.
FIG. 1 is a block diagram of a conventional apparatus for such surface treatment. The conventional apparatus includes a spin chuck 22 for holding a central portion of a light transmitting wafer 21 by suction and for rotating the wafer 21 around its center. A full cone nozzle 23 is provided above the wafer 21. The nozzle 23 sprays a treatment solution in a conical pattern onto the upper surface of the wafer 21. A projection fiber 26 is provided under the wafer 21, opposite to the lower surface of the wafer 21. The fiber 26 projects light onto the lower surface of the wafer 21. The conventional apparatus further includes a light emitting device 25 for applying light to the projection fiber 26. A light receiving fiber 27 is provided above the wafer 21, opposite to the projection fiber 26. The fiber 27 receives light projected from the projection fiber 26. The conventional apparatus further includes a light receiving device 28 for receiving light from the light receiving fiber 27 and for outputting a photoelectric signal by photoelectric conversion. The conventional apparatus further includes a treatment end detecting portion 29 for detecting an end point of surface treatment in response to the photocurrent from the light receiving device 28.
Operation of the conventional apparatus is disclosed in Japanese Patent Laying-Open No. 62-144332, entitled "Aluminum Spin Spray Etching Apparatus".
The conventional apparatus operates in the following manner. The spin chuck 22 maintains the wafer 21 in a horizontal position by supporting the center of the lower surface of the wafer 21 by a suction mechanism (not shown). A layer 30 to be treated is formed in advance on the upper surface of the wafer 21. The layer 30 includes, for example, a metal layer (not shown) to be etched, and an etching resist mask (not shown) formed on the metal layer with a desired pattern.
A drive means (not shown) rotates the spin chuck 22 at a predetermined angular speed such that the wafer 21 rotates around its center. The nozzle 23 sprays a treatment solution (e.g., an etchant) onto the upper surface of the wafer 21. The nozzle 23 sprays the treatment solution in a conical pattern. The portion of the metal layer which is not protected by the mask is etched by the etchant.
Light generated by the light emitting device 25 is projected onto the lower surface of the wafer 21 through the projection fiber 26. The light receiving fiber 27 is opposed to the projection fiber 26. The wafer 21 is located therebetween. Light projected from the projection fiber 26 is transmitted through the wafer 21 and the layer 30 to be treated and reaches the light receiving fiber 27. At this point, a first component of light from the fiber 26 reaches the fiber 27. The remaining component is reflected downward by the upper surface of the metal layer, is further reflected upward by the upper surface of the wafer 21 and then reaches the light receiving fiber 27.
The difference between the optical paths of the two components is equal to twice the thickness of the metal layer. Since the components follow different optical paths, the components interfere with each other. The metal layer of the layer 30 to be treated is decreased by etching. Thus, the difference between the optical paths become smaller as etching proceeds. The point in time when the portion of the metal layer which is not protected by the mask is completely etched is called the etching end point. The etching end point is defined as the point in time when the intensity of the light which is transmitted through the wafer 21 does not change due to interference between its components.
The light receiving device 28 converts light received by the light receiving fiber 27 to a photoelectric signal. The treatment end detecting portion 29 monitors changes in the photocurrent and when the changes in the photocurrent become smaller than a predetermined value and stable, a determination is made to discontinue etching. Such automatic detection of the etching end point enhances repeatability and improves precision.
The treatment solution is sprayed in a conical pattern onto the surface of the wafer by the full cone nozzle because this is believed to provide the most uniform treatment. However, actually, the sprayed treatment solution is applied in the form of a large number of particles, which collide with the surface of the wafer. As a result of such collision, a small unevenness or a rugged pattern might be formed on the surface of the wafer. Such unevenness might adversely affect the treatment of the surface of the wafer. For instance, an oxide film formed on the rough surface is liable to be electrically broken.
In addition, in the conventional apparatus, detection of the end point of treatment is often inaccurate. One of the causes of such inaccuracy is an unstable state of the optical paths. This instability can be caused by vibrations in the treatment solution layer due to non-uniformity of the treatment solution on the wafer surface. The non-uniformity can be caused by spraying the treatment solution.
Another cause of inaccuracy is centrifugal diffusion of the treatment solution due to the rotation of the wafer. Such diffusion causes non-uniformity of the treatment solution. The non-uniformity of the treatment solution causes the amount of light reaching the fiber 27 to vary widely. This creates noise which makes it difficult to detect the end point of treatment with accuracy. In other words, the amount of photoelectric signal inputted to the detecting portion 29 varies by factors other than light interference.
It is particularly difficult to accurately detect the end point of treatment when etching contact holes having a very small diameter. Normally, an opening ratio of the contact holes (the ratio between the area of the surface of the wafer and the total area of all the contact holes formed in the wafer) is less than 5%. Because of this small opening ratio, there is little change in the intensity of light as etching proceeds. Furthermore, treatment solution crosses the light path with irregularity and, as a result, an irregular change occurs in the intensity of light reaching the light receiving fiber 27. These unfavorable conditions make it difficult to accurately detect the end point of treatment.